Fiber-optic communications systems employ techniques known as wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) to transit large volumes of information at high data rates over a single optical fiber. Many optical wavelengths of light or “optical channels” are employed in both WDM and DWDM systems, with each optical channel individually transmitting a substantial amount of data. The optical power of one optical channel co-propagates with a number of the other optical channels within a single optical fiber.
It is often necessary to remove or add an optical channel to an optical fiber depending on routing requirements of the communications systems and for other reasons. This function is accomplished by a filter, typically referred to as a drop/add filter or an interleaver/de-interleaver. There are different types of drop/add filters disclosed in the prior art, but the general principal of operation involves the introduction of a wavelength division multiplexed or dense wavelength division multiplexed signal into the input port of the filter. The signal includes a number of different optical channels, as noted above. In performing the “drop” function, the filter is operative to remove at least one of the optical channels from the signal and transfer it to a drop port while allowing the remainder of the signal to pass through the filter to an output port. The “add” function of the filter is realized by introducing a signal with one or more optical channels into the filter through an add port where it is combined within the filter with another signal from the input port. The combined signal is then transmitted to the output port.
One type of optical filtering device currently in use is an in-fiber Bragg grating placed in series with an optical circulator. A wavelength division multiplexed signal or dense wavelength division multiplexed signal is input to the circulator and the Bragg grating is effective to allow certain optical channels of the signal to pass through while one or more others are reflected back to the circulator and output through a drop port. These types of filtering systems have excellent spectral performance, but are large and have significant throughput loss.
Another type of filtering system is shown, for example, in U.S. Pat. No. 6,580,851 to Vahala et al which describes a four-port optical filter employing one or more tapered optical fibers coupled to a spherical resonator. A wavelength division multiplexed signal is input through one optical fiber to the spherical resonator by evanescent field coupling. The resonator is effective to drop one or more optical channels from the signal, or add an optical channel, which is then output from the resonator to the same optical fiber or a second optical fiber. Filtering systems of this type require precise alignment between the resonator and tapered optical fiber(s), and spherical resonators are difficult to manufacture. Additionally, this system has a high optical Q. The term “optical Q” refers to the quality factor of the resonator in the system, and high Q resonators exhibit a much narrower filter response as compared to low Q resonators. A broadband filter response is preferable in channel interleavers and de-interleavers for dense wavelength division multiplexing.